The invention relates to pumps, pump components, and pumping methods, particularly high pressure piston pumps of the type where a slipper is located between the piston and a drive member. Pumps of this type may be used to pressurize engine oil used in a Hydraulic Electronic Unit Injector (HEUI) diesel engine fuel system.
Slipper type piston pumps are well known. In these pumps a piston is fitted in a piston bore and is moved back and forth along the bore by a cylindrical eccentric on a crankshaft. A slipper is located between the piston and the eccentric and is held against the eccentric by a spring in the bore. The slipper has a partial cylindrical surface that engages the eccentric and a recess that receives an end of the piston. Retraction of the piston during an inlet stroke draws fluid into the pumping chamber. Extension of the piston along a pumping stroke flows pumped fluid from the assembly, typically past a spring backed check valve.
In these pumps the pistons are commonly made of hardened steel and the slippers are made of softer bronze. The spherical end of the piston and the spherical recess in the bronze that receives the piston end are carefully manufactured to exacting tolerances in order to assure proper engagement between the piston and the slipper. The thickness of the oil film between the spherical surfaces is taken into account in sizing the spherical surfaces. Manufacture of pistons and slippers with exactly mating spherical surfaces is expensive and difficult. Failure to manufacture the pistons and slippers with mating surfaces increases wear.
Diesel engines using HEUI fuel injectors are well known. A HEUI injector includes an actuation solenoid which, in response to a signal from the diesel engine electronic control module, opens a valve for an interval to permit high pressure engine oil supplied to the injector to extend a fuel plunger and inject fuel into the combustion chamber.
HEUI injectors are actuated by oil drawn from the sump of the diesel engine by the diesel engine oil pump and flowed to a high pressure pump assembly driven by the diesel engine. The pump assembly pumps engine oil at high pressure into an oil manifold or compression chamber. The manifold or chamber is connected to the HEUI injectors. Except for large engines, the high pressure pump assembly typically includes a swash plate pump using axial pistons and having an output dependent upon the speed of the diesel engine. The pistons have spherical ends that engage spherical slippers with flat faces. The slippers and pistons are extended and retracted by rotation of a cylinder barrel containing the piston bores. The flat faces of the slippers bear and slide against a flat swash plate at a fixed angle with respect to the axis of rotation of the cylinder barrel. Large engines sometimes use a variable angle swash plate pump where the output can be varied independently of engine speed.
In conventional swash plate pumps the pistons are made of hardened steel and the slippers are made of a softer material, typically bronze. The spherical surface on the inner end of each piston has a radius only slightly smaller than the radius of the spherical surface in the slipper to permit maintenance of an oil film between the piston and slipper as the slipper moves angularly relative to the piston during each pumping stroke. Friction, lubrication, and wear between the spherical surface of the piston and the spherical surface of the slipper are complex phenomena, commonly described as contact between the piston and slipper spherical surfaces, although the surfaces are separated by an oil film.
Manufacture of precisely matched spherical surfaces in conventional swash plate pumps is typically accomplished by deforming the softer slipper spherical surface to conform to the harder spherical surface of the piston. Pistons and slippers with spherical surfaces that do not match within the thickness of an oil film have high bearing contact pressure and experience high wear.
Therefore, there is a need for an improved high pressure pump, pump components and method. The pump, pump components and method are particularly useful in a HEUI diesel engine but are also useful in other types of pumps and pumping applications. A pump according to the invention used in a HEUI diesel engine can pump engine oil into a high pressure oil manifold or chamber in a variable amount sufficient to maintain the desired instantaneous pressure in the manifold without substantial overpumping. In a HEUI system, return of pressurized high pressure oil to the sump should be minimized to avoid unnecessary energy loss.
The invention is an improved slipper type high pressure pump; components for a slipper type pump and method for operating a slipper type pump.
The pump is useful in pressurizing fluid, particularly oil used to actuate HEUI fuel injectors for diesel engines. The high pressure pump includes a crank which reciprocates pistons in bores. A slipper is positioned between the crank and pistons. A spring in the piston bore keeps a spherical end of the piston in a slipper recess and keeps the slipper against the crank. The piston is hardened steel and the slipper is formed from bronze, a material softer than hardened steel. The slipper end of the piston is spherical and extends into a specially shaped, nearly spherical recess formed in the top of the slipper. This recess has a radius of curvature greater than the radius of curvature of the piston end and has an opening at the top of the slipper that is larger than the piston diameter.
When the piston is first seated in the recess in the slipper the spherical surface on the piston engages the surface in the slipper at a circular line of engagement. During initial operation of the pump the pressure exerted on the slipper by the piston during pumping at the narrow line contact deforms the softer bronze to increase the area of contact and form a wider circular band. The circular band has sufficient width to support the piston without additional deformation.
The spherical surface on the end of the piston and the near spherical surface on the slipper reduce the cost of manufacturing the piston and slipper. Both the surfaces may be manufactured with dimensional tolerances greater than the tolerances required for matching the radii of the pistons and slipper with an allowance for an oil film.
The pump includes a crankshaft having two spaced cylindrical eccentrics with each eccentric driving two separate slipper type piston pumps. In each pump, fluid flows through an unobstructed inlet passage extending from an inlet throttle valve through a crank chamber surrounding the crank, through the eccentric and through openings in the slippers and pistons and into the pumping chamber to fill the pumping chamber during return strokes. During pumping strokes the inlet passage through the slipper is closed and the piston is moved through a pumping stroke to pressurize the fluid in the pumping chamber and flow the pressurized fluid past check valve and from the pump. On both pumps, the inlet passages into the pumping chambers are unobstructed during return strokes of the pumps to facilitate filling when the pumped fluid does not flow readily, typically when the fluid is cold and viscous. This feature is important in HEUI pumping systems during startup of diesel engines when the engine oil is cold and viscous and must be drawn from a reservoir at engine crankcase pressure before lube oil pressure at the inlet builds up.